For an image forming apparatus, such as a printer or a copier, because of demands for a reduction in a rise time (time from power-on of an image forming apparatus to a state in which a fixing device becomes able to fix an image) and energy saving, and other demands, attention is directed to a fixing device that uses a heating rotary belt capable of having small thermal capacity. Attention is also directed to a induction-heating fixing device that includes an induction heating portion as means for heating a heating rotary belt at a location that faces the outer surface of the heating rotary belt and heats the heating rotary belt by causing the heat generation layer of the heating rotary belt to generate heat using an electromagnetic induction action produced by a magnetic flux generated by an induction coil (magnetic-flux generating portion) of the induction heating portion.
For an induction-heating fixing device, the positional relationship between the outer circumferential surface of a heating rotary belt and an induction coil may be changed by thermal expansion of a pressure rotary roller. In this case, the heating efficiency of the fixing device varies and is uneven, power consumption tends to increase, and the rise time of the fixing device is apt to vary.
Another proposed example of a fixing device is the one that positions the outer circumferential surface of a heating rotary roller with respect to an induction heating portion to reduce a change in the positional relationship between the heating rotary roller and an induction coil. The proposed fixing device has a configuration in which the induction heating portion is urged toward a bearing portion of the heating rotary roller by an urging member arranged between the induction heating portion and the bearing portion of the heating rotary roller, and the induction heating portion is positioned.
Unfortunately, the fixing device using the heating rotary belt capable of having small thermal capacity does not have a bearing portion as in the proposed fixing device, so it is difficult to urge the induction heating portion toward the heating rotary belt and position the induction heating portion. Such difficulty in positioning may lead to uneven heating efficiencies of the heating rotary belt in the direction of the rotation axis of the heating rotary belt (i.e., the paper width direction) or variations in heating efficiency between produced fixing devices. In the former, the temperature is not even in the direction of the rotation axis of the heating rotary belt; in the latter, the fixing devices have different rise times. Thus, it is desired that a fixing device be capable of having a uniform distance between an induction coil and the outer circumferential surface of a heating rotary belt.